Hey,

thanks for your answers, I read the articles and was able to solve a half of the actual problem, which is the following:

I have the function

El[k_ , l_, d_] := k * l / (0.25*d^2*Pi)

and computed the gaussian variance formula

Eld [k_, l_, d_, kd_, ld_, dd_] := 
 Sqrt[kd (Derivative[1, 0, 0][El])^2 + 
   ld (Derivative[1, 0, 0][El])^2 + dd (Derivative[1, 0, 0][El])^2]

It works pretty well for El... El[1,2,3] gives me 0.282942.

However, Eld[1, 2, 3, 4, 5, 6] doesn't work, which makes sense because Eld can't give k,l and d to El. However, I do not know how to solve this problem. Can you help me again?

Thank you,

Tobias

We don't know what you want to do, so you might either need = or := depending on what you want to do. If a, b and, c are numbers, the derivative does not make any sense, you can't do the derivate with respect to a number...

I think you want this:

ClearAll[f, g]
f[a_, b_, c_] := a + b/c
g[a_, b_, c_] = D[f[a, b, c], a]

you can now check the definition of g:

??g

giving

g[a_,b_,c_]=1

as expected...